<!DOCTYPE HTML>
<html style="background:white">
<body>
<canvas id="b" style="position:absolute; left:0; top:0" width="20" height="20"></canvas>
<div style="position:absolute; left:0; top:0; -moz-transform:rotate(45deg); height:400px; width:400px; opacity:0.5; border:1px solid black;"></div>
<div style="position:absolute; left:-3px; top:-3px; -moz-transform:rotate(45deg); height:390px; width:390px; border:10px solid red;"></div>
<script>
function fillCanvas(id) {
  var ctx = document.getElementById(id).getContext("2d");
  ctx.fillStyle = "black";
  ctx.fillRect(0, 0, 10, 10);
}
fillCanvas("b");
</script>
</body>
</html>
